Type 2 diabetes mellitus (T2DM) is getting epidemic threatening millions of people and its increase is tightly related with the rapid increase of obesity throughout the world. Insulin resistance may proceed decades before the onset of diabetes and it is common in obese individuals. My long term goal in research is finding primary cause(s) of insulin resistance and elucidating glucose-fat metabolic interactions in T2DM, which are essential for strategy development to prevent the onset of diabetes and to find a cure or better treatments for the disease. A key clinical observation for the diagnosis of diabetes is fasting hyperglycemia. Hepatic glucose overproduction contributes to fasting hyperglycemia, but controversial data have been reported about the roles of glycogenolysis and gluconeogenesis in T2DM. Recently I found that preserved liver glycogen in fasting resulted in excess glycogenolysis, and subsequently contributed to fasting hyperglycemia in the rodent models of obesity-associated T2DM. This observation could be important in understanding the failure of hepatic glucose auto-regulation in obese T2DM patients because increased hepatic glycogen in fasting was reported in obese humans and T2DM patients. The excess liver glycogen could be critical in fasting hyperglycemia because endogenous glucose production is sensitive to the amount of hepatic glycogen available for hydrolysis. In this proposal, liver metabolic changes in obesity and in obesity-associated T2DM will be evaluated in animal models and in human subjects. Hepatic insulin resistance induced by short high-fat diet and glucose-fat metabolic interaction in obesity-associated T2DM will be evaluated focusing hepatic glycogen. It will be determined if the inhibition of excess glycogenolysis improves fasting hyperglycemia in obesity-associated T2DM. The Advanced Imaging Research Center has pioneered in method development to evaluate comprehensive metabolic fluxes in vivo and to study intermediary metabolism using simultaneous administration of stable isotope tracers. The technologies with high field nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance (MR) spectroscopy will be adapted in performing proposed research projects.